This invention relates to auxiliary lift axle wheel assemblies for load carrying vehicles, in which an auxiliary axle and wheel assembly carried by the vehicle can be moved down toward load-bearing engagement with the road surface when the vehicle is heavily loaded, or can be lifted upwardly away from the road surface when there is little or no load carried by the vehicle.
The prior art includes lift axle wheel assemblies of various arrangements whereby air inflatable bags or "air springs" positioned between the auxiliary axle and vehicle body are used to force the auxiliary axle downward relative to the vehicle body into engagement with the road surface. In some of the prior art structure such as shown in U.S. Pat. No. 3,730,549, the inflatable air bags are positioned in direct contact with the axle, that is approximately atop the axle. In other prior art, such as U.S. Pat. No. 3,771,812, the auxiliary lift axle is supported at its ends by levers each pivotally mounted to the vehicle at one end, the axle mounted intermediate the ends of the levers, and the inflatable air bags positioned over the other ends of the levers. In both prior art structures, inflating the air bags places a downward force on the auxiliary lift axle, moving it downward relative to the vehicle body toward load bearing engagement with the road surface.
When it is not necessary to use the auxiliary lift axle, it should be returned to its "up" position and held there away from contact with the road surface to avoid unnecessary wear on its wheels. In some of the prior art lift axle wheel assemblies leaf springs have been used to lift the auxiliary axle when the air springs are deflated. The leaf spring assembly is connected at its mid-point to the axle with its forces directed upward against the air bags, thus holding the axle up until the spring force is overcome by the force of inflating bags.
Coil springs also have been used in some prior art lift axle wheel assemblies to lift the axle. For example, U.S. Pat. No. 3,771,812 illustrates a coil tension spring which is expanded by downward motion of the axle. When the downward force exerted by the air bags is removed, the spring contracts and lifts the axle to its up position.
In most of the known prior art lift axle wheel assemblies the springs used to lift the axle have been large and cumbersome and the springs have been exposed to the dirt and other road contaminants which, in some instances, are deleterious to the spring structure.